1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for controlling generation of a constant current using a selectable gain.
2. Description of the Related Art
Many electronic systems utilize a direct current (DC) to supply power to a load. “DC current” is also referred to as “constant current”. A light emitting diode (LED) represents one example of a constant current load. Light emitting diodes (LEDs) are referred to as constant current devices because LEDs utilize a DC current power source to provide light. References herein to “constant current” refer to a direct current (DC). “Constant” current does not mean that the current cannot change over time. The DC value of the constant current can change to another DC value. Additionally, a constant current may have noise or other minor fluctuations that cause the DC value of the current to fluctuate. “Constant current devices” have a steady state output that depends upon the DC value of the current supplied to the devices.
LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output, long life, and environmental incentives such as the reduction of mercury. LEDs are semiconductor devices and are best driven by direct current. The brightness of the LED varies in direct proportion to the DC current supplied to the LED. Thus, increasing current supplied to an LED increases the brightness of the LED and decreasing current supplied to the LED dims the LED.
FIG. 1 depicts power distribution system 100 that converts power from voltage source 102 into power usable by load 104. Load 104 is a constant current load that includes, for example, one or more LEDs. A controller 106 controls the power conversion process. Voltage source 102 supplies an alternating current (AC) input voltage VIN to a full bridge diode rectifier 108. The voltage source 102 is, for example, a public utility, and the AC voltage VIN is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The rectifier 108 supplies a rectified AC voltage VX to the switching power converter 110. The switching power converter 110 serves as a power supply that converts the AC voltage VX into a DC link voltage VLINK.
The controller 106 provides a control signal CS0 to switching power converter 110 to control the conversion of rectified input voltage VX into a link voltage VLINK. The switching power converter 110 can be any type of switching power converter, such as a boost, buck, boost-buck, or Cúk type switching power converter. The link voltage VLINK is generally a DC voltage that is maintained at an approximately constant level by switching power converter 110. Controller 106 also generates control signal CS1 to control load drive switch 112. When control signal CS1 causes switch 112 to conduct, a link current iLINK flows into a primary coil 114 of transformer 116 to magnetize the primary coil 114. When control signal CS1 opens switch 112, primary coil 114 demagnetizes. The magnetization and demagnetization of the primary coil 114 induces a secondary voltage VS across a secondary coil 118 of transformer 116. Primary voltage VP is N times the secondary voltage VS, i.e. VP=N·VS, and “N” is a ratio of coil turns in the primary coil 114 to the coil turns in the secondary coil 118. The load current iLOAD is a direct function of the secondary voltage VS and the impedance of diode 120, capacitor 122, and load 104. Diode 120 allows the load current iLOAD to flow in one direction. The load current iLOAD charges capacitor 120, and capacitor 120 maintains an approximately DC voltage VLOAD across load 104. Thus, load current iLOAD is a DC current.
Since the control signal CS1 generated by the controller 106 controls the link current iLINK, and the link current iLINK controls the voltage VP across the primary coil 114, the energy transfer from the primary coil 114 to the secondary coil 118 is controlled by the controller 106. Thus, the controller 106 controls the load current iLOAD.
The controller 106 operates the switching power converter 110 in a certain mode, such as quasi-resonant mode. In quasi-resonant mode, the control signal CS1 turns switch 112 ON at a point in time that attempts to minimize the voltage across switch 112, and, thus, minimize current through switch 112. Controller 106 generates the control signal CS1 in accordance with a sensed link current iLINK—SENSE, obtained via link current sense path 126. However, the value of the sensed link current iLINK—SENSE used to determine a period of control signal CS1 is largely dictated by components of power distribution system 100 whose values are fixed. Conversely, the value of the components is generally fixed by values of the period of control signal CS1 that are used to operate switch 112. Thus, the controller 106 does not have much flexibility in the determination of the period of control signal CS1, and providers of components of power distribution system 100 have limited flexibility in specifying the values of the components.